Such devices exhibit operating frequencies of the order of a few hundred MHz to a few GHz, and are used in radiofrequency transmission circuits (portable telephone, radio link, wireless data exchange, etc.), for signal processing or in sensor systems.
More precisely the field of the invention is that of bulk wave, also dubbed “BAW” for “Bulk Acoustic Wave”, devices.
A BAW resonator can serve notably to produce bandpass filters beyond a gigahertz for mobile telephone applications for example. Usually, the piezoelectric material is deposited by techniques such as cathodic sputtering. An alternative to deposition is the transfer of monocrystalline layers, this possibly being employed to improve the properties of such resonators.
The advantage is multifold: these materials intrinsically comprise fewer losses (compared with aluminium nitride AlN or zinc oxide ZnO obtained by depositions), can have much greater couplings and, depending on the crystalline orientation, the value of the coupling can be chosen.
The benefits of the transfer of piezoelectric monocrystalline layers for acoustic applications are notably presented in publications by NGK insulator and as described in the article: Y. Osugi, T. Yoshino, K. Suzuki, T. Hirai, Microwave Symp., 873 (2007) as well as in the articles by M. Pijolat, J. S. Moulet, A. Reinhardt, E. Defaÿ, C. Deguet, D. Gachon, B. Ghyselen, M. Aïd, S. Ballandras, IEEE Ultras. Symp., 201 (2008), or in the following articles by J. S. Moulet, M. Pijolat, J. Dechamp, F. Mazen, A. Tauzin, F. Rieutord, A. Reinhardt, E. Defaÿ, C. Deguet, B. Ghyselen, L. Clavelier, M. Aïd, S. Ballandras, C. Mazuré, IEEE IEDM, (2008) or else by M. Pijolat, D. Mercier, A. Reinhardt, E. Defay, C. Deguet, M. Aid, J. S. Moulet, B. Ghyselen, S. Ballandras, IEEE Freq. Contr. Symp., 290 (2009).
Two types of technologies have been described in patents, making it possible to obtain resonators decoupled from the effect of the substrate, i.e. to manufacture bulk wave devices in suspended films also dubbed “Film Bulk Acoustic Resonators” or “FBARs” as well as “Solidly Mounted Resonators” or “SMRs”, exhibiting structures such as those illustrated respectively in FIGS. 1a and 1b. The former comprise suspended thin layers of piezoelectric material 1 inserted between two electrodes 2 and 3 on the surface of a substrate S in which a suspended structure S0 has been produced, the latter can comprise for example Bragg mirrors MR making it possible to confine the bulk waves and thus to decouple them from the effect of the substrate S.
The first technology described in patents US20070200458 and US20070210878A1, consists of bonding followed by thinning of an LiNbO3 substrate down to a micron, the active devices resting on air gaps. Such thinning to below a micron, while preserving great homogeneity thickness-wise during thinning, is very difficult.
The second technology described in patent U.S. Pat. No. 6,767,749 presents various methods for obtaining a monocrystalline thin layer acoustically decoupled from the substrate. This technique is based on the implantation of gaseous ions (typically hydrogen) termed the implantation/fracture technique, the implantation/fracture method making it possible for its part to achieve very low thicknesses that may typically be less than about 0.5 μm but which are limited in respect of greater values to 2 μm with a microelectronics conventional implanter and to a few tens of μm with high-energy implanters. The method of transfer by implantation/fracture is described in detail in the article: “Silicon on insulator material technology”, Electronic letters, 31 (14), p 1201-1202 (1995), it allows notably the production of SOI “Silicon On Insulator” substrates. It is used in this patent to transfer the said layer onto a Bragg reflector type stack or else onto cavities.
It is the latter solution which is deemed very difficult because of the technologies to be employed for transfer. A stiffening material is required in order to ensure transfer atop the cavities and may be removed later. During the bonding step, a vacuum must be made in the cavities so as to avoid problems with overpressure during subsequent temperature rise, a delicate step requiring special bonding machines.
The inherent problem with these two technologies remains the difficulty of obtaining the suspended thin layer.